1. Field of the Invention
The present invention relates to a process to co-process renewable feedstock materials with a gas to liquids process wherein at least a portion of hydrogen used to process the renewable materials is produced in a gas to liquids process.
2. Prior Art
Mixtures of triglycerides and fatty acids can be hydroprocessed to produce chemicals and fuels, such as jet and diesel fuel. Sources of renewable feed can be seed oils, crop oils, animal fats, recycled greases and oils including soy oil, jatropha, camalina, palm oil, yellow grease, and other natural materials. The processing of these materials requires a considerable volume of hydrogen. By-products include CO2, water, propane and other light hydrocarbons. These natural feed materials are generally considered to be a renewable resource and are increasingly desirable sources to produce fuels in a sustainable manner. The production and use of fuels made from these resources also result in a very low production of greenhouse gases.
Triglyceride feeds have been converted to fuels via a transesterification reaction with methanol to make biodiesel for many years. This process is becoming less desirable as it produces a lower quality fuel. These same renewable feedstocks can be converted to high quality fuels by processing with hydrogen over a catalyst. This has been practiced as a standalone operation or by co-processing in a conventional refinery.
The present invention provides a novel process to convert renewable feed materials to fuels and chemicals by integrating operations with a gas to liquids process.
A gas to liquids (“GTL”) process integrates several process steps to convert natural gas into fuels and/or chemicals.
First, natural gas is reacted with steam and/or oxygen to produce synthesis gas comprising carbon monoxide and hydrogen. This is a high temperature reaction involving a complex series of reforming and combustion reactions. This step is typically catalytic and may be performed in a steam methane reformer (“SMR”) or an autothermal reformer (“ATR”). This step can also be accomplished non-catalytically in a partial oxidation reactor. As will be described herein in detail, a preferred method of the present invention is to use an ATR.
The synthesis gas produced in the first step is cooled and cleaned before further use. This may also include adjusting the H2:CO ratio to accommodate downstream requirements.
The second step in the GTL process is conversion of the synthesis gas to hydrocarbon products. This is typically a Fischer Tropsch reaction carried out over an iron based or cobalt based catalyst. The reactor can take a number of forms, including a fixed bed, fluidized bed, ebullating bed, microchannel or slurry bubble column reactor. The catalyst and reactor must be carefully matched to account for synthesis gas and product concentrations and heat transfer limitations to operate at the desired performance.
The third step of the GTL process is to upgrade the raw hydrocarbon products from the Fischer Tropsch reactor to produce one or more products that meet a defined specification. For example, if a middle distillate fuel is the desired product, it could be refined to meet ASTM D-975 specifications.
The foregoing three key steps of the GTL process are integrated with utilities such as oxygen production, power generation, water treating, steam production and hydrogen management to meet the objectives of a specific plant design. The GTL process also requires additional infrastructure, such as safety systems, flares, tanks, loading facilities to transport products and maintenance facilities, etc.
An objective of the present invention is to use the GTL process to leverage production of renewable products. This can be done with any GTL process, but a preferred embodiment is to utilize small modular GTL. Small, as used herein, is defined as between 500 to 5,000 barrels per day (“BPD”). The reason for this preference is the relative size of typical renewable feedstock options. Renewable materials are typically available in limited quantities at any given location, for example, 500 to 1,000 BPD. For a small, modular GTL plant, this is a good fit and the co-processing of the renewable feed can be advantageous to the GTL plant. For large 50,000 to 100,000 BPD plants, the relatively small volumes of renewable materials are too small to make a significant contribution to the plant production.
There is currently a good deal of interest in production of renewable fuels. One method of producing renewable fuels is to gasify a raw renewable carbonaceous material to produce synthesis gas and use a Fischer Tropsch reaction to produce hydrocarbon products for upgrading. The chemistry of this process is similar to a GTL process, but the process is much more complex. In the case of a renewable biomass feed, it is typically an irregular shaped solid material that must be stored on site, fed by auger or screw into a gasifier, which typically operates at lower pressure. The resulting synthesis gas must then be compressed and can have a variety of contaminants, such as sulfur and halogen compounds, minerals, tars, particulates and ash, that must be removed to very low levels required by the Fischer Tropsch catalyst. By comparison, natural gas flows into a GTL plant under pressure. It has a well-defined composition with relatively few contaminants and commercial proven methods for cleanup. Commercial SMR's and ATR's have demonstrated operation at intermediate pressure (20-40 bars), which is ideal for Fischer Tropsch operation without further compression.
Another method to produce renewable fuels is to hydroprocess a renewable fat or oil, such as crop oils, animal fats, algae oils or a polymer such as recycled plastics. These materials can be processed with hydrogen to reduce the oxygen content and/or molecular weight, thus deriving a marketable hydrocarbon product. One of the challenges for processing these materials is the availability of hydrogen. Hydrogen is expensive to produce and requires significant infrastructure.
The objective of the present invention is to utilize the infrastructure and resources of a GTL process, preferably a small modular GTL process to co-process renewable feedstocks to provide an integrated fuel processing system. Four key elements of the integrated process result in improved efficiency and economics:                1. Hydrogen from the GTL process—a typical SMR produces a synthesis gas with a H2:CO ratio of 3:1. A typical ATR produces a ratio of 2.2 to 2.6:1. Methane as a feedstock has a very high H2:CO ratio (2:1) compared to a biomass feed, which is typically closer to 1:1. Therefore, the synthesis gas obtained by conversion of natural gas by the best available technology and the highest efficiency possible will typically have excess H2 greater than the 2:1 ratio required by the Fischer Tropsch reaction. This hydrogen can be removed and purified for downstream processing, such as upgrading Fischer Tropsch products and co-processing of renewable products.        2. GTL infrastructure—a GTL plant will have a significant amount of utilities and infrastructure that can be leveraged for co-processing a renewable feed. If the renewable feed is processed in a standalone plant, significant infrastructure would be required. Most of this infrastructure is already in place with a GTL plant with few additions required for the incremental co-processing load.        3. Light product gas utilization—hydroprocessing of the renewable feed results in loss of product as the glycerides are decomposed into CO, CO2, H2O and light hydrocarbons. In a standalone plant producing fuels from these feeds, the light gas products cannot be recycled. Some of the light gases can be collected as liquefied petroleum gas (“LPG”), but it has lower value. In the integrated process of the present invention, these light gas products, which can be as much as 15% of the feed, can be recycled to the reformer for production of synthesis gas which can then be converted to liquid products. This renewable material now becomes part of the Fischer Tropsch product.        4. Reduced CO2 footprint—the integrated process of the present invention results in a reduced level of CO2 added to the atmosphere for the volume of products produced. By recycling waste products from the hydroprocessing of the renewable feed, a portion of the synthesis gas, and hence a portion of the Fischer Tropsch products, are based on carbon from the renewable source. While the Fischer Tropsch products are predominately made from natural gas, the renewable content could be as high as 40% when using a SMR to generate the synthesis gas. The Fischer Tropsch and renewable products both require further processing with hydrogen to make finished products.        
The upgrading of these products can be done together or separate. The Fischer Tropsch products to be upgraded typically include paraffin hydrocarbons from C5 to C100. The long chain products C21+ can be hydrocracked to middle distillate fuels or can be hydroprocessed to make solvents, waxes or lube base oils. For waxes and base oils, it is desirable to minimize cracking, in which case the heavy Fischer Tropsch products will be upgraded separately. If the target is middle distillate fuels, the heavy Fischer Tropsch waxy products (C21+) may be co-processed with the renewable feed. In either case, the hydroprocessed product can be blended and distilled or kept separate and distilled into finished products. The final product upgrading configuration is defined by the product target specifications. The finished products, whether derived from natural gas or renewable feed, will be totally compatible and can be blended in any proportion with each other or with other petroleum derived products. Products that are co-processed may result in renewable content of from 1% to 80%. When the renewable products are processed separately, the renewable content of those products is 100%. However, the separately processed GTL products will still have a small renewable content of 1% to 40% due to utilization of the light gases, which are not part of the desired product slate, from the renewable materials that are recycled to make additional synthesis gas.
The broad range of potential renewable content in the products is based on the range of excess hydrogen available, depending on the configuration of the reforming section of the GTL plant and the ratio of renewable feedstock to Fischer Tropsch derived hydrocarbons available for hydroprocessing. A SMR can be operated efficiently to produce synthesis gas in an approximately 3:1 H2:CO ratio. The Fischer Tropsch reaction requires synthesis gas in approximately 2:1 ratio. Therefore, in the case of a SMR, there is substantial potential for excess hydrogen. This excess hydrogen is typically recycled and used as fuel in the SMR, but could be used for downstream hydroprocessing, resulting in a substantial volume of hydrogen available for hydroprocessing. In this case, the amount of renewable feed could result in approximately three times as much renewable product as Fischer Tropsch product. Theoretically, the SMR synthesis gas could be shifted to all hydrogen and then the synthesis gas plant would be strictly a hydrogen source that could process 100% renewable feed. That configuration is not part of the scope of the present invention.
The objective of the present invention is to efficiently utilize a GTL plant to leverage additional production of renewable feedstocks. The advantage to the GTL plant is to reduce the CO2 footprint of the plant and utilize the infrastructure and hydrogen of the plant to produce additional products. This includes leveraging the light gas products from hydrodeoxygenating a renewable feed to produce additional synthesis gas for the Fischer Tropsch reaction, such products also being of a renewable nature. In the case of an ATR, the amount of excess hydrogen is much less, resulting in a renewable feed limit approximately equal to the Fischer Tropsch production. The renewable feed could also be less, depending on availability, hence the broad range. With the SMR being the practical limit of the present invention for excess hydrogen available for downstream processing and assuming the renewable feedstock is not limiting, the maximum renewable content of a blended product is approximately 80%. If the products are hydroprocessed separately, the Fischer Tropsch derived products could have approximately 40% renewable content due to recycling of light gas components.
Co-processing of the renewable feeds has been proposed by Mackay et al. (U.S. Patent Publication No. 2011/0113676). In this reference, municipal solid waste (“MSW”) is the primary feed. The MSW is refined to produce a refuse derived fuel (“RDF”) that is depleted of inorganics. The RDF is gasified to make synthesis gas. The synthesis gas is converted to a Fischer Tropsch raw product. Excess hydrogen is used to upgrade a combined Fischer Tropsch product and a triglyceride feed.
The present invention differs from the Mackay reference in that it is based on reforming natural gas. Natural gas by nature has a high hydrogen to carbon ratio. The result is that efficiently reforming the gas provides a H2:CO ratio greater than required by the Fischer Tropsch reaction. The excess hydrogen can efficiently be utilized for downstream processing without sacrificing efficiency in the GTL portion of the plant. In the case of MSW, as with most biomass resources, the nature of the feed is deficient in hydrogen. Therefore, the gasifier is operated with excess water in the feed to produce a higher H2:CO ratio. While there may be operational advantages to the high water feed, it is not optimum from a carbon standpoint, as more CO2 will be produced in order to make hydrogen not only for the renewable section of the plant, but also to close the gap between the low H2 content in the MSW to the ratio of about 2:1 needed for the Fischer Tropsch process. Also, while the hydrogen derived from natural gas is not renewable, it is much easier to produce since the contaminant level in natural gas is significantly less than MSW. In the case of natural gas reforming, the reformer can operate at 20-40 bars, sufficient to pass directly to the Fischer Tropsch reactor without further compression. Processing MSW at these pressures is costly and inefficient.
The Mackay process does not take advantage of recycling waste products from hydroprocessing of renewable feeds. The Mackay process also does not find advantage to the reduced CO2 footprint enjoyed by the present invention, as the nature of the Mackay primary feed is considered to be renewable.
A type of co-processing of renewable feeds is taught by Knuuttila (U.S. Patent Publication No. 2010/0317903). In this reference, a biological feed is gasified to make synthesis gas. The synthesis gas is converted to Fischer Tropsch hydrocarbon products. The Fischer Tropsch hydrocarbon product is hydroprocessed and a separate bio oil is also hydroprocessed. The Fischer Tropsch products and hydroprocessed bio oils are combined and fractionated.
The present invention differs from the Knuuttila reference in that synthesis gas is generated by reforming natural gas, whereas Knuuttila gasifies biomass. In the present invention, excess hydrogen is efficiently extracted from the synthesis gas for downstream hydroprocessing, whereas Knuuttila produces hydrogen in a separate reformer by reforming waste components or imported methanol, recognizing that the biomass feed stream is deficient in hydrogen. The Knuuttila design gains no advantage of reduced carbon footprint enjoyed by the present invention, as it utilizes biological feed.
Gasification of a carbonaceous feed is taught by Blevins et al. (U.S. Patent Publication No. 2011/0178185). While the carbonaceous feed is clearly directed at renewable biomass, natural gas is included in the definition of carbonaceous feed. Unlike the present invention, this reference does not teach efficient extraction of hydrogen for downstream processing of renewable feeds or co-processing of such streams.
The present invention is directed to a process to efficiently utilize resources, such as hydrogen and infrastructure of a gas to liquids process, to efficiently co-process renewable fats and oils or polymers. Such processing enhances the GTL operation by reducing the carbon footprint, adding a small amount of renewable material into the Fischer Tropsch product and adding a new renewable product in a very capital and energy efficient manner.
A purpose and object of the present invention is to provide an integrated fuel processing system which has advantages over either a natural gas to liquids process or a biomass hydroprocessing process.